The invention relates to the creation and detection of nuclear magnetic resonance. More particularly the invention relates to the application of nuclear magnetic resonance in an angular rate sensor or gyroscope.
A number of approaches have been suggested in the prior art for creating a nuclear magnetic resonance gyroscope. In general, they use a nuclear magnetic resonance controlled oscillator and rotational information is derived from the phases of the nuclear moment Larmor precession signals by phase comparison and magnetic field control circuits.
These devices have significant problems which limit their use. For instance, certain devices are limited by the relatively short relaxation times of the gases which they use. Also, typical strong direct coupling between the gases and the light which is used for magnetic moment alignment or magnetic moment detection limits both the relaxation times and the signal-to-noise ratio, and therefore limits usefulness of such instruments.
In U.S. Pat. No. 4,157,495 a nuclear magnetic resonance (hereinafter referred to as "NMR") angular rate sensor or gyroscope is disclosed that operates by sensing inertial angular rotation rate or angular displacement about a sensitive axis of the device as a shift in the Larmor precession frequency or phase, respectively, of one or more isotopes that possess nuclear magnetic moments. The gyroscope has an angular rotation sensor and its associated electronics. The principal elements of the sensor are a light source, an NMR cell, a photodetector, a set of magnetic shields and a set of magnetic field coils. The principal elements of the electronics are signal processing circuits to extract the Larmor precession frequency and phase information and circuits to generate and control magnetic fields, both steady and varying sinusoidally with time, that are used for operating the device.
The NMR cell is mounted within a set of magnetic shields to attenuate external magnetic fields to acceptably low levels. Magnetic field coils are used to apply very uniform magnetic fields to the NMR cell. Both a steady field and an AC carrier field are applied along the sensitive axis of the device, and AC feedback fields are applied along one of the transverse axes. The DC magnetic fields along both transverse axes are controlled to be substantially zero. The NMR cell contains an alkali metal vapor, such as rubidium or potassium, together with two isotopes of one or more noble gases, such as krypton-83, xenon-129, or xenon 131. A buffer gas such as helium may also be contained in the cell.
The NMR cell is illuminated by a beam of circularly polarized light, from a source such as a rubidium or potassium lamp or a rubidium or potassium solid state laser, which passes through the cell at an angle with respect to the steady magnetic field. Absorption of some of this light causes the atomic magnetic moments of the rubidium or potassium atoms to be partly aligned in the direction of the steady magnetic field. This alignment is partly transferred to the nuclear magnetic moments of the noble gases, and these moments are caused to precess about the direction of the steady magnetic field, which in turn creates magnetic fields that rotate at the respective Larmor precession frequencies of the two noble gases. The rotating fields modulate the precessional motions of the rubidium or potassium magnetic moments, which produces corresponding modulations of the transmitted light, to make it possible optically to detect the Larmor precession frequencies of the two noble gases.
The modulations of the light intensity are converted into electrical signals by a photodetector, and these signals are then electronically demodulated and filtered to create signals at the Larmor precession frequencies of the two noble gases. The difference between the two precession frequencies is used accurately to control the steady magnetic field so that it is constant. One of the noble gas precession frequencies is compared to a precision reference frequency, and the resulting difference frequency is a measure of the angular rotation rate of the gyroscope.
The two detected noble gas precession signals are also used to generate two AC feedback magnetic fields at the Larmor precession frequencies of the noble gases, and these are responsible for sustaining the precession of the nuclear magnetic moments of the noble gases. The use of an AC carrier magnetic field facilitates the optical detection of the precessing noble gas moments, and it is used to control the DC magnetic fields along the two transverse axes of the gyroscope.
According to the patent, the NMR gyroscope includes means for the simultaneous alignment of the nuclear magnetic moments of at least two nuclear moment gases which constitutes a nuclear magnetic moment alignment device; means for achieving sustained precession of these moments which constitutes a nuclear magnetic resonance oscillator capable of sustained oscillations; means for the optical detection of these precessing nuclear moments which constitutes a nuclear magnetic resonance detection device; means for accurately controlling the internal magnetic field of the device; and means for the accurate measurement of the frequency or phase of the detected nuclear moment precession signal of at least one of the nuclear moment gases to provide a measurement of the angular rotation rate or angular displacement, respectively, of the device with respect to inertial space.
More particularly, a steady magnetic field is applied to an NMR cell which is substantially shielded from other steady magnetic fields. The NMR cell contains a gas or vapor of a substance that possesses a magnetic moment that can be aligned by optical pumping, together with one or more additional gases, each of which possesses a nuclear magnetic moment. The NMR cell is illuminated by optical pumping light which has a directional component which is parallel to the direction of the steady magnetic field and which has the proper wavelength to be absorbed by the optically pumpable substance to align the magnetic moments of that substance. The nuclear moments of the nuclear moment gases are aligned and precess at their respective Larmor precession frequencies about the direction of the steady magnetic field. An AC magnetic field at a suitable carrier frequency is also applied to the NMR cell, and the cell is illuminated by detection light which has a directional component that is orthogonal to the direction of the AC carrier magnetic field and which has a wavelength that is essentially the same as that of the optical pumping light. The intensity of the part of the detection light that is transmitted by the cell is modulated in response to the total magnetic fields present in the cell including the magnetic fields that are generated by the precessing nuclear magnetic moments. The modulations of the transmitted light intensity are detected by a photodetector, and the detected signals are electronically demodulated to obtain signals at the Larmor precession frequencies of the nuclear moment gases.
In one embodiment of the patented invention, the alignment of the nuclear magnetic moments of each nuclear moment gas is accomplished by collisional interactions between the atoms of the optically pumpable substance and the atoms of the nuclear moment gas or gases. Sustained precession of the nuclear magnetic moments of each nuclear moment gas is produced by an AC feedback magnetic field at the Larmor precession frequency of the nuclear moment gas oriented a direction that is orthogonal to the direction of the steady magnetic field. The AC carrier magnetic field has a frequency at substantially the Larmor precession frequency of the optically pumpable substance, and it is directed substantially parallel to the direction of the steady magnetic field to permit the device to be operated at higher values of steady magnetic field strength and at correspondingly higher Larmor precession frequencies for the nuclear moment gases.
In the preferred embodiment of the patent, an optically pumpable substance such as a single alkali metal vapor is placed in an NMR cell together with two noble gases, and the nuclear magnetic moments of both noble gases are aligned simultaneously by collisional interactions between the atoms of the alkali metal atoms and the atoms of the two noble gases. The alkali metal is rubidium or potassium, and the noble gases are xenon-129, and xenon 131.
Another feature of the patent involves the use of at least one buffer gas in substantial quantities in the NMR cell.
In still another feature of the patent, the magnitude of the steady magnetic field remains constant because of feedback control of the field to cause the difference between the Larmor precession frequencies of the two noble gases in the NMR cell to be equal to a predetermined constant value.
It is yet another feature of the patent that one of the Larmor precession frequencies is compared to a precision reference frequency, and the resulting difference frequency is used as a measure of angular displacement or angular rate of the device about the direction of the steady magnetic field.